The present invention pertains to a system for effecting an order change in the stacking system of a corrugator and, more particularly, to a method for effectively handling small gap order changes.
In a corrugator dry end, a corrugated paperboard web is longitudinally scored and slit into multiple parallel output webs (or "outs"), and the outs are directed through one or more downstream cut-off knives which cut the output webs into selected sheet lengths. The sheets are then directed into a variable speed stacking system where the sheets are compressed into a shingle and delivered into a downstacker where a vertical stack of sheets is formed for discharge. Order changes must be effected while the upstream corrugator wet end continues to produce and deliver the continuous web to the dry end. An order change will typically require repositioning of the slitter-scorer and a change in the sheet length provided by the cut-off knife or knives.
In order to accommodate repositioning of the slitting and scoring tools for an order change, two basic types of order change systems have been developed in the corrugated industry. One of the order change systems is known as a gap style system. A gap system uses a rotary shear positioned immediately downstream of the corrugator wet end. At order change, the rotary shear is operated to make a cross cut through the entire web. The downstream dry end equipment is accelerated to pull a gap between the tail edge of the old running order and the leading edge of the new order defined by the shear cut. As the tail edge of the web passes through the slitter-scorer, the slitting and scoring tools are repositioned in the gap and set for the new order.
The other system is known as a gapless or plunge style order change system. In this system, there are two sets of slitting and scoring tools immediately adjacent one another in the direction of web movement and through both of which the corrugated web travels. At order change, one slitter-scorer, operating on the currently running order, will lift out of operative engagement with the web, and the other slitter-scorer which is set to the new order alignment plunges down into operative engagement with the web. The result is a small order change region of corrugated web with overlapping slits and scores for both the old order and the new order.
In U.S. Pat. No. 5,496,431, a laterally adjustable cutting tool is positioned over the center of the web and makes a running diagonal cut to provide a transition in the widths of the outs between the old and new orders. In this region, the slitter-scorer for the old order is withdrawn and the slitter-scorer for the new order is plunged into the web. The diagonal pieces which are formed to provide the gapless order change cannot be discharged in the usual manner onto the downstream stacks of corrugated board. Thus, the board pieces exiting the downstream cut-off knife containing the diagonal connecting slit and the overlapping slit and score lines require the use of a separate diverter downstream of each cut-off knife to divert the resultant scrap sheets.
In German Patent 44 25 155, alternately operable plunge cut slitter-scorers are utilized, but the overlapping slits and scores from the old and new orders are removed by positioning a separate rotary shear and scrap sheet diverter between the slitter-scorer and the cut-off knife.
In all of the foregoing order change systems, a gap is eventually created between the corrugated board forming the old and new orders. The gap may be formed upstream of the slitter-scorer, between the slitter-scorer and the cut-off knife, or after the cut-off knife. As a result of the various order change systems and depending also on corrugator line speed, the gap may be larger than 2 seconds in time or as short as 0.2 second. In the stacking system, immediately downstream of the cut-off knife, the sheets are shingled in a process which necessarily requires the sheets to be slowed to create the overlap in the shingle. In a manner known in the art, the stacking system typically includes a series of variable speed conveyors, including an upstream shingling conveyor receiving sheets directly from the cut-off knife. At order change, the old order shingle is separated from the sheets of the new order by accelerating the stacker conveyors, directing the old order sheets into the stacker, and sequentially slowing the stacker conveyors in the downstream direction with passage of the tail end of the old order, in a manner described in U.S. Pat. No. 4,200,276.
At order change, where the gap between the tail edge of the last sheet of the old order and the lead edge of the first sheets of the new order is large (e.g. 2 seconds), the shingled old order is able to clear the vacuum shingling conveyor before the first sheet of the new order (being fed at higher line speed) overtakes the old order. Increasing corrugator and line speeds, up to and above 1,000 feet per minute (300 m/min), have led to a number of modifications in stacker systems. The delivery of relatively short sheets at high speeds led to the development of two stage shingling by positioning two adjacent vacuum shinglers at the upstream end of the stacking conveyor system. Sheets are thus preshingled at a somewhat higher speed (e.g. 50% of line speed) and immediately reshingled at a lower speed (e.g. 25% of line speed) for discharge into the downstacker.
If an old order is followed by a new order of substantially longer length sheets (e.g. 200 inches or about 5 m), a relatively small gap between the orders may result in the lead edge of the long first sheet of the new order overtaking the tail edge of the last sheet of the old order while the latter is still in the shingling section of the stacker system. The result is socalled "edge butt" where the higher speed overrunning new order sheet hits the last sheet of the old order causing it to be knocked out of position on the shingle with consequent misfeed and jamming.
One prior art method for preventing edge butt involves stopping the entire stacker conveyor system immediately upon the tail edge of the last sheet of the old order being captured by the upstream end of the first vacuum conveyor. Because the tail end of the first vacuum shingling conveyor is positioned vertically below the line speed input nip (e.g. 1.5 inches or about 40 mm), the first sheet of the new order will override the tail edge of the last sheet of the old order, the shingle will be reestablished, whereupon the stacker system conveyors are restarted and the old and new orders separated as the tail edge of the former moves past the downstream end of the second vacuum shingling conveyor.
It has now been discovered that, where the old order sheets are relatively short, such as about 24 inches (about 0.6 m), the new order sheets are significantly longer, such as about 60 to about 200 inches (about 1.5 to 5 m), and the gap between orders is small, such as about 0.5 second or less, reestablishing the shingle and allowing the first new order sheet to override the old order causes another problem. The long new order sheet, because it overruns a number of sheets at the tail end of the old order shingle, imposes a normal force on the tail end of the shingle such that, at the downstream separation point, the tail end of the old order shingle cannot be satisfactorily pulled from beneath the long overriding sheet. As a result, a number of short old order sheets are typically left behind, disrupting the sheet count and the discharge process.